Remove unnecessary check in SymbolFileDWARF::ParseImportedModules

Summary:
This check seems unnecessary as we already assert the same condition above and also access `sc.comp_unit`
before this check.

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: jdoerfert, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61394

llvm-svn: 359813